1. Field of the Disclosure
This disclosure relates generally to apparatus and methods for determining a distance to a pre-existing wellbore and controlling drilling operations based on the determination.
2. Background of the Art
In the process of drilling wells (also referred to as wellbores or boreholes) for hydrocarbon production, it is common to drill a second well in a predetermined relationship to an existing well. An example of this may be when a blowout occurred in the existing well. Two approaches may be taken to control the blowout. One method is to use explosives at the surface and snuff out the fire in the burning well. This procedure is fraught with danger and requires prompt control of hydrocarbons flow in the well. The second method is to drill a second borehole to intersect the blowout well and pump drilling mud into the blowout well. This is not a trivial matter. An error of half a degree can result in a deviation of close to 90 feet at a depth of 10,000 feet. A typical borehole is about 12 inches in diameter, a miniscule target compared to the potential error zone.
Another situation in which accurate placement of a secondary well relative to a preexisting well is desired for secondary recovery operations. For various reasons, such as low formation pressure or high viscosity of hydrocarbons in the reservoir, production under natural conditions of hydrocarbons may be at uneconomically low rates. In such cases, a second borehole is drilled to be substantially parallel to the pre-existing borehole. Fluid such as water, CO2 is then injected into the formation from the second borehole and the injected fluid drives the hydrocarbons in the formation towards the producing borehole where it may be recovered.
In the second category are passive ranging techniques that do not require access to the pre-existing well while drilling the second well. The techniques normally utilize a relatively strong magnetism induced in the casing of the pre-existing well by the Earth's magnetic field. The signal due directly to the earth's magnetic field is a problem, limiting the accuracy of this measurement. Residual magnetism of the casing introduces additional uncertainties.
The present disclosure discloses apparatus and methods for determining distance from a pre-existing wellbore without accessing the pre-existing well.